1. Field of the Invention
The present invention relates to a spring steel superior in fatigue properties. More particularly, the present invention relates to a spring steel (for valve springs and the like) superior in fatigue properties which is characterized by a low content of undeformable inclusions.
2. Description of the Related Art
It is a well-known fact that such steels as used for valve springs, which need a high fatigue strength, are liable to fracture that starts from hard non-metallic inclusions if they contain them. In order to prevent fracture due to such hard inclusions, there have been proposed several ways to control their composition such that they have a melting point lower than 1500.degree. C. Such hard inclusions are made smaller by hot or cold rolling or drawing.
There is disclosed a steel with a high degree of cleanliness in Japanese Patent Publication No. 74484/1994. This steel contains non-metallic inclusions whose average composition is SiO.sub.2 : 20.about.60%, MnO: 10.about.80%, and CaO: 13.about.50%, and/or MgO: 5.about.15%. These non-metallic inclusions are characterized by that the ratio of length (l) to width (d) measured in the longitudinal cross-section of rolled steel is l/d.ltoreq.5. There is also disclosed a steel with a high degree of cleanliness in Japanese Patent Publication No. 74485/1994. This steel contains non-metallic inclusions whose average composition is SiO.sub.2 : 35.about.75%, MnO: Al.sub.2 O.sub.3 : .ltoreq.30%, CaO: 10.about.50%, and MgO: 3.about.25%. These non-metallic inclusions are characterized by that the ratio of length (l) to width (d) measured in the longitudinal cross-section of rolled steel is l/d.ltoreq.5.
The steels with a high degree of cleanliness disclosed in the above-mentioned patents are designed to improve the fatigue properties by controlling the average composition of non-metallic inclusions such that the ratio of length (l) to width (d) is l/d.ltoreq.5. The present inventors found that they have the following problems.
Even though the average composition of non-metallic inclusions is controlled so that l/d.ltoreq.5, there still exist hard inclusions exceeding this limit and they cause breakage. Moreover, even though inclusions are ductile and satisfy the condition l/d.ltoreq.5, they also cause breakage if they are thick.